Recently, the broadcast wave for television in Japan is being changed from an analog terrestrial broadcasting having a VHF band ranging from 90 to 220 MHz or a UHF band ranging from 470 to 770 MHz to a digital terrestrial broadcasting ranging from 470 to 710 MHz. A device for transmitting and receiving airwaves in a vehicle also needs to deal with the digital terrestrial broadcasting.
Previously, an antenna pattern has been constructed with a central focus on a horizontal element in many cases. In a case that such a antenna pattern is provided to a front window glass or a rear window glass of the vehicle, this antenna is apparently viewed as a dot or short-line shape by airwaves coming from a side direction (lateral side) of the vehicle. Hence, there has been a problem that a sufficient resonance cannot be obtained so that a receiving sensitivity for the airwaves coming from the side direction of the vehicle is low to cause an insufficient directivity.
As a countermeasure, if an antenna is mounted on a side window glass of the vehicle to complement the reception of the airwaves coming from the side direction by diversity reception, the directivity is improved to a satisfactory level. However, in this case, a place to which the antenna pattern is provided is limited to a non-opening window glass of the vehicle. Hence, in a type of vehicle to which the previous antenna pattern cannot be provided such as a vehicle type in which the side window does not include a non-opening window glass having an area sufficient to form the antenna pattern and a vehicle type in which the non-opening window is located in a part of a door, a plurality of antennas are provided to the front window glass and the rear window glass to perform the diversity reception. However, in this case, a satisfactory directivity is not obtained to the airwaves coming from the side direction of the vehicle.
Moreover, since the broadcast wave for television is nowadays being changed from the analog terrestrial broadcasting having the VHF band or the UHF band ranging from 470 to 770 MHz to the digital terrestrial broadcasting ranging from 470 to 710 MHz, the device for transmitting and receiving airwaves in a vehicle needs to deal with the digital terrestrial broadcasting. For the transmission and reception of the digital terrestrial broadcasting, an antenna having higher sensitivity than that for the transmission and reception of the analog terrestrial broadcasting is necessary, and an ungrounded-type antenna system needs to be employed. Since this ungrounded-type antenna system tends to occupy a large area as compared with a grounded-type antenna, it has been necessary to reduce the area of the ungrounded-type antenna.
Japanese Patent Application Publication No. 2005-354139 discloses one concrete example in which an antenna for receiving the digital broadcasting is provided to a window glass of vehicle. In this technique, a glass antenna apparatus for vehicle receives signals for digital television (maximum frequency: f1, its corresponding wavelength: λ1, minimum frequency: f2, its corresponding wavelength: λ2). This glass antenna includes two of an upper-side antenna element and a lower-side antenna element which are provided to the window glass of the vehicle to satisfy a positional relation in upper-lower direction with each other and both of which extend in the horizontal direction; and includes a phase synthesizer which retards one of reception signals derived from power feeding terminals of these two antenna elements by a phase of 190°˜220° and which combines the retarded reception signal with another reception signal. (see Patent Literature 1)
Moreover, Japanese Patent Application Publication No. 2002-100914 discloses another technique of glass antenna for a vehicle. In this technique, a first antenna conductor, a second antenna conductor, a power-feeding point for the first antenna conductor and a power-feeding point for the second antenna conductor are provided to a window glass plate of the vehicle. The first antenna conductor intersects with the second antenna conductor. At this intersection portion, the first antenna conductor and the second antenna conductor are formed in a layered structure so as to sandwich an insulation layer therebetween. The first antenna conductor intersects with the second antenna conductor to form an intersection angle ranging from 60 degrees to 120 degrees. A length L1 of the first antenna conductor and a length L2 of the second antenna conductor satisfy a relation of 0.5·L2≦L1 2.0·L2. (see Patent Literature 2)
Moreover, Japanese Patent Application Publication No. 7-46016 discloses an ungrounded-type window glass antenna for an automobile telephone. In this technique, a hot-side antenna line is provided to a window glass and extended in the horizontal direction so as to be electromagnetically coupled with an upper or lower horizontal edge of a metallic window frame of vehicle body to which the window glass is attached. On the other hand, an earth-side antenna line is located in an inner (central) portion of the window glass beyond the hot-side antenna line. One end of the earth-side antenna line is located near one end or a center of the hot-side antenna line. (see Patent Literature 3)
Furthermore, Japanese Patent Application Publication No. 2009-49706 discloses a glass antenna for a vehicle as an ungrounded-type antenna for digital terrestrial broadcasting. In this technique, the antenna for receiving a broadcast wave for the digital terrestrial television is provided to a corner portion of a window glass of a side portion of the vehicle that is located on a rear-seat side. This antenna includes two positive and negative (power-) feeding points provided to the corner portion of the side window glass, a first element, and a second element. The first element includes a first vertical line which extends vertically toward a corner and which is at least one of at least two lines extending from the positive feeding point, and a first horizontal line extending horizontally from a tip of the first vertical line or from an intermediate portion of the first vertical line in a branched manner. The second element includes at least two second horizontal lines extending horizontally from the negative feeding point, and a second vertical line extending vertically from the negative feeding point. The positive and negative feeding points are connected respectively with a core wire and an enveloping conductor wire of a coaxial cable. (see Patent Literature 4)